It has become common practice to house power distribution cables in conduits to isolate and protect the cables from elements which might damage them or cause them to deteriorate. In practice, the conduits are fashioned to direct the cables to desired locations and are literally built into the system, or structure, or building. The cables are installed later by running lead lines through the conduits and utilizing these lead lines to pull the cables into place in their respective conduits.
Running a lead line through a lengthy conduit with numerous bends therein can be a difficult task. One of the most successful practices that has been devised for this purpose involves pulling a lead line into the conduit behind a pulling plug (also sometimes called a "birdie" or a "rabbit") which is propelled through the conduit by compressed air introduced into the conduit behind the plug. This practice has been known at least as early as Mar. 11, 1890 when U.S. Pat. No. 423,134 was granted to T. J. Cope for "LAYING ELECTRICAL CONDUCTORS".
The apparatus devised by T. J. Cope and subsequently improved on by others employed members which were manually held against the entry end of the conduit to seal the conduit so that air pressure could be built up therein to propel the pulling plug. U.S. Pat. Nos. 3,301,531, granted Jan. 31, 1967, to R. J. Corsiglia for "APPARATUS FOR INSTALLING ELECTRICAL WIRE IN ELECTRICAL CONDUIT" and 3,793,732, granted Feb. 26, 1974 to J. C. Hamrick for "METHOD AND APPARATUS FOR INSTALLING HEAVY ELECTRICAL CONDUCTOR LINES IN CONDUITS" disclose apparatus of this character. Such apparatus was capable of establishing only a very low air pressure within the conduit and consequently the pulling plugs were capable of pulling only lightweight lead lines. Such lead wires were too fragile to be used to directly pull heavy electrical power distribution cables through the conduits. The rather laborious practice was introduced of pulling a succession of lead lines of increasing diameter and strength until one was pulled through that had sufficient strength to in turn pull the electrical cable through.
More recently it has come to be recognized that if the apparatus employed to plug the entry end of the conduit could be mechanically affixed to the conduit it would be capable of maintaining a higher air pressure within the conduit. The higher air pressure, in turn, could drive the pulling plug with greater force and enable it to pull a heavier weight, stronger lead line. The pulling of lighter and intermediate weight lead lines could thus be eliminated. The device for mechanically closing the entry end of a conduit and having provisions for admitting both air under pressure and a lead line to the conduit is hereinafter referred to as a "blow plug".
U.S. Pat. No. 4,382,581, granted May 10, 1983 to D. W. Wimer et al. for "APPARATUS FOR PULLING CABLE THROUGH UNDERGROUND CONDUIT" discloses one such blow plug intended to facilitate the pulling of heavier lead lines. Wimer et al. propose the use of an "air gun" having an expandable rubber ring which constitutes the sole means for sealing the end of the conduit and for mechanically holding the gun in the conduit. It can be readily appreciated that the rubber ring could conceivably slip within the smooth bore of a conduit. The reliability of the Wimer et al. air gun consequently is questionable. In addition, the Wimer et al. gun employs a rather complex trap door arrangement to permit the lead line to be fed into and through the gun.
There is a continuing need, therefore, for a blow plug which is both reliable and easy to use.